Suspension systems provide vibration isolation from one part of the vehicle to another. Suspension systems also position a sprung mass relative to an unsprung mass. The vibration producing portion of the vehicle is generally termed the unsprung mass and the section being isolated is called the sprung mass. Vibration isolation may be achieved, in part, by converting high frequency vibration into lower frequency motion of greater amplitude. However, high frequency vibration cannot be converted in an unlimited manner to relatively large amplitude, low frequency movement without seriously compromising the objective of maintaining the preferred spacing of the sprung and unsprung masses.
One example of a system of sprung and unsprung masses are the chassis and the wheels of a motor vehicle. Similarly, the driver and his seat can be isolated from the chassis of the vehicle. Under normal driving conditions, a driver should be kept at a nearly fixed position relative to the controls of the vehicle, and, under extreme conditions, within reach of the controls. However, the objective of keeping the driver properly positioned is not consistent with the object of providing a comfortable ride. Passive and semi-active suspension systems provide for decay of the movement of the sprung mass, typically by use of a friction based damping element such as a viscous fluid shock absorber. Active suspension systems force the return of the sprung mass to a desired, equilibrium position relative to the unsprung mass.
Passive suspension systems usually comprise mechanical springs and viscous fluid or friction shock absorbers for damping movement of the sprung mass relative to the unsprung mass. Such systems have the virtue of simplicity, but the disadvantage of being optimized for a particular frequency. When used for a seat where the Occupant's mass can easily vary from 40 kg. to 150 kg., that frequency may not even be particularly predictable.
Semi-active suspension systems use a spring and a friction type motion damping device. They differ from passive systems in that they provide control of the damping rate of the shock absorber. See for example, U.S. Pat. Nos. 5,582,385 and 5,259,487. By using condition detecting sensors and a microprocessor, control of the damping rate of the shock absorber may be made dynamic.
Active suspension systems require the use of sensors and provide dynamic adaptation to sensed conditions. While active suspension systems typically have a spring they do not have a friction type a motion damping device. Instead, active suspension systems dynamically control the total force applied between sprung and unsprung masses to provide for quickly returning the masses to a predetermined spacing. U.S. Pat. No. 4,892,328 teaches a strut for a vehicle primary suspension combining a mechanical spring and an electromagnetic positioning element. The electromagnetic positioning element of the '328 patent is constructed to provide positioning force axially along a strut like assembly, with the force level proportional to the current supplied and the direction of the force determined by Be diction of the current in the electromagnet. The current applied to the winding is dynamically varied as a function both of linear displacement between the masses and external forces applied along the shaft and may serve to push apart the masses or draw them together.
Passive, semi-active and active suspension systems each have potential application to seat suspension systems. The dynamic response characteristics of the semi-active suspension system of U.S. Pat. No. 5,652,704 allowed the incorporation of additional features in such seat suspension systems, such as automatically deflating air springs upon egress of the driver.
There is frequently a safety advantage to vehicle occupants if their seat position is changed duringa collision or rollover. Generally, an occupant is safer if occupant movement toward the front or the top of the passenger compartment or cab is restrained or if pinning of the occupants in the vehicle can be prevented. Floor mounted belts, if in use by the driver or occupant, partially restrict movement of the seat by squeezing the driver into the seat when the seat moves upward and forward relative to the vehicle during a collision. However, interaction between the belt and upward movement of the seat can contribute to the driver "submarining", that is, the driver being pulled partly below the dash.
U.S. Pat. No. 5,344,204 to Liu discloses a collision detection sensor which operates to release a seat locking mechanism so that compressed springs can force the seat backward toward the rear of the vehicle. The system of Liu is not integrated with the existing seat suspension and as a result adds to the mechanical complexity of the installation.